Various proposals have heretofore been made on microcapsules consisting of a core material and a shell covering the core material. Among these proposals, microcapsules whose capsule shell have been formed by interfacial polymerization are excellent in the completeness of covering of the core material and the inner retention and some of them have been put into practical use, e.g., non-carbon paper and pressure measuring paper. In these uses, the microcapsule is applied to a support such as paper with a proper binder resin. Thus, microcapsules particles are used in the form of suspension in the binder resin. However, if microcapsule particles are used in the form of independent powder, it is difficult to keep a volatile liquid in the core substance over a prolonged period of time because the capsule obtained by interfacial polymerization has a low mechanical strength and normally has a shell thickness of not more than 0.5 .mu.m. In the production of interfacial polymerization type microcapsule, a method is normally employed which comprises using a low boiling solvent along with the core substance (see JP-A-56-119137 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-58-145964, JP-A-63-163373, JP-A-64-40949, "New Microcapsulization Technology and Examples of Development of Its Application", Microcapsule Kenkyukai, pp. 50-52, Keiei Kaihatsu Center, September 1978, Tamotsu Kondo, Masumi Koishi, "Microcapsule", pp. 30-32, Sankyo Shuppan, November 1987). In some detail, an oily composition comprising a core substance, a capsule shell-forming monomer, a low boiling solvent, and optionally other additives is emulsified in an aqueous medium. The emulsion is then capsulized while the low boiling solvent being removed from the oily droplets. The low boiling solvent present in the oily droplets serves not only to lower the viscosity of the oily composition to facilitate emulsification but also to cause the capsule shell-forming monomer to migrate to the interface with the droplets to accelerate capsulization reaction. In accordance with this method, microcapsules can be normally obtained having a better mechanical strength and core substance retention than those obtained free of low boiling solvent.
However, this method is disadvantageous in that the low boiling solvent cannot be recovered. In this method, the reaction is allowed to proceed by evaporating the low boiling solvent from the reaction system to the atmosphere. If the low boiling solvent is recovered by distillation under reduced pressure, the reaction solution suffers from violent foaming, making it extremely difficult to recover the solvent. Further, the evaporation of the low boiling solvent to the atmosphere not only adds to production cost but also worsens the environmental protection, safety and sanitation. Moreover, in the case where capsulization is effected while the low boiling solvent being evaporated to the atmosphere, the reaction must be effected over a prolonged period of time to fully remove the low boiling solvent. If the low boiling solvent remains in the core, it causes a great problem when the microcapsule is used as a toner. In some detail, the solvent remaining in the core exudes out to the surface of the capsule and thus deteriorates the fluidity of the toner, resulting in the deterioration of chargeability and hence developability of the toner. The exudation of the solvent also causes the modification of the photoreceptor. Solvents having a relatively higher boiling point and a lower water solubility can remain in the core more remarkably.
In the case where the microcapsule is used as a toner, it is more difficult to assure both mechanical strength and core substance retention. Various proposals have heretofore been made on microcapsule toners comprising a capsule shell covering a core substance. For example, JP-A-54-66844, JP-A-55-18630, JP-A-57-41647, and JP-A-57-202547 disclose the use of a wax compound as a core substance. JP-A-52-108134, JP-A-58-9153, JP-A-59-159174, and JP-A-59-159177 disclose the use of a soft polymer as a core substance. Further, JP-A-56-119137, JP-A-58-145964, and JP-A-63-163373 disclose an interfacial polymerization type microcapsule toner comprising a polymer solution as a fixing component for core substance. Among these proposals, the interfacial polymerization type microcapsule toner comprising a polymer solution as a core component has an extremely excellent fixability but can hardly maintain a high boiling solvent in the polymer solution in the core substance. Further, the foregoing microcapsule toner can hardly maintain a sufficient mechanical strength without impairing the fixability thereof.
Since it has been believed that in a process for producing a microcapsule using cellulose dispersion stabilizer as a dispersant, the dispersion stabilizer undergoes gelation at an elevated temperature higher than gelation temperature to cause lowering of capsule strength. Therefore, the elevated temperature has not been used in the process. Further, in the prior arts, the stabilizer is not set in a form of gelation, but in a form of solution. Accordingly, when a low boiling solvent contained in a capsule is excluded from the system at a polymerizing step, foams are generated in the solution of the dispersant, i.e., paste-like solution, to be a foam-solution. Thereby, it has been difficult to recover the solvent contained in the foam-solution.
While, it has been found by the inventors that since the stabilizer is effective at only the initial stage of dispersion of the oily droplets into water, an interface polymerization takes place immediately at the interface of oily phase and hydrophilic phase to form polymer film (outer shell), after the dispersion once has been completed. Accordingly, it has been also found that the stabilizer is allowed to act in a minimized degree after formation of polymer film. As a result, it has been also found that the environmental temperature for polymerization higher than the temperature of gelation leads to providing rice grain-like gel of the dispersant in water, and thereby the low boiling solvent which is released from the water at the polymerization step is liable to be recovered under cooling.